1. Field of the Invention
The present invention relates generally to a high frequency heating apparatus having an output controlling function and, more particularly, to a high frequency heating apparatus where a breakdown of the switching element and noise generated due to frequency change are both prevented.
2. Description of the Prior Art
In a conventional high frequency heating apparatus, a reference voltage generator which outputs a consumption power change signal corresponding to a plurality of predetermined high frequency outputs is constructed as follows:
As shown in FIG. 1, bases of transistors TR3-TR5 are connected to output ports P1-P3 of a microprocessor through resistors R11-R13, and collectors of transistors TR3-TR5 are connected to a non-inverting input terminal of a ground negative feedback amplifier with resistors R14-R17 and a condenser C3. The following describes the operating relationship of the apparatus of FIG. 1.
First, consumption power change signals are outputted from the microprocessor output ports according to the user's preference in eight varieties, as shown in FIG. 2. If the respective "low", "high", "low" signals from output ports P1-P3 of a microprocessor are outputted, transistors TR3, TR5 turn on and transistor TR4 turns off. This causes a voltage VO to be both inputted to the non-inverting input of a negative feedback amplifier, and outputted from the negative feedback amplifier, and outputted from the negative feedback amplifier, according to Formula (1) below: ##EQU1##
If all "high" signals are outputted from the microprocessor, transistors TR3-TR5 turn off. Therefore, the output voltage VO becomes 0 V (zero volts). However, when transistors TR3-TR5 turn off, the voltage VO reduces exponentially when the charge of condenser C3 discharges through resistor R17. As the output voltage VO thus reduces slowly, an abnormal noise according to the frequency change is generated. If a plurality of output control terminals exist, there have been problems associated with the requirement that there be an increased plurality of output control ports of the microprocessor, which only adds to the adverse noise generation.
In U.S. Pat. Nos. 4,920,246, 4,593,167, 4,835,353, 4,833,581, 4,931,609, 4,900,885, there are disclosed conventional high frequency heating apparatuses related to the present invention. Of these, one representative example of a high frequency heating apparatus is the one described in U.S. Pat. No. 4,920,246. It comprises a high frequency heating source for providing a predetermined high frequency heating output, an invertor means including a rectifier for rectifying an AC input power, and a switching means for switching a DC voltage from the rectifier and providing a high frequency drive power source for driving the high frequency heating source; a voltage detection means for detecting output voltage values from switching elements of the invertor means, a microprocessing means for outputting an on-off signal where the determined generating timing depends on interrupt processing according to output voltages detected by the voltage detection means. The microprocessor includes a generating means for generating a reference clock, a counter means for counting the reference clock generated by the generating means according to a setting value related to a high frequency heating output, and on-off signal generating means for outputting on-off signals having respective on-off periods according to the counting result of the counter means.
Furthermore, this apparatus operates in an output control mode at a particular on-off time duty ratio (duty cycle) by using a microprocessor having a plurality of control signals. It detects an input current by use of a current transformer on a terminal of the AC input power source, feeds the current back to the microprocessor, and outputs a control signal through the output port by controlling the output quantities via the microprocessor. The apparatus is thus controlled by use of a portion of the AC input power source. However, there have been the following problems in this and other such apparatuses of the prior art. A control signal for driving a switching element is directly outputted by the microprocessor. Therefore, if the microprocessor itself generates error due to noise, etc., a high signal for driving a switching element is continuously outputted from an output port of the microprocessor. In such a case, the switching element may break down since more than the predetermined high voltage is applied to it.